The present invention relates to electrical power distribution systems, and more particularly, to the control of bus tie breakers in electrical power distribution systems.
Conventional electrical power systems architecture, such as those used in aerospace applications, usually needs to be reconfigured in the event of a power source failure or interconnect cabling failures. In electrical power systems having multiple power sources, when one power source fails, power may be transferred to the distribution bus of the failed power source from another power source. Bus Tie Contactors (BTCs) are typically used to accomplish this transfer of power between busses fed by different sources of electrical power.
FIG. 1 shows a conventional electrical power distribution system 10 in accordance with the prior art. FIG. 1 shows the electrical power system 10 during normal mode of operation. Two electrical power sources 12, 14 are connected to electrical loads 16, 18 via distribution buses 24 and 26 located in power distribution panels 20, 22 respectively. The power sources 12, 14 may be, for example, electrical generators. Power bus bars 24, 26 connect the electrical power sources to the plurality of electrical loads 16, 18 through a plurality of Electrical Load Control Units, e.g., (ELCUs) 28, 30. The ELCUs 28, as well as circuit breakers (not shown) may be used to provide line protection for each load 16, 18.
In the electrical power distribution system 10 bus tie contactors (BTCs) 32, 34 are used to allow transfer of, or to isolate electrical power between, power bus bars 24, 26. The transfer may be performed by connecting (“tying”) electrical power buses together through electrical connection 36, which may comprise a cable. The BTCs 32, 34 may be used to reconfigure the system under certain fault conditions to ensure the availability of power on both buses 24 and 26.
Electrical power system 10 may be a variable frequency or a constant frequency power system. In a variable frequency power system, the power sources 12, 14 are not synchronized and power buses cannot be tied together. During normal operation, as shown in FIG. 1, each power source 12, 14 delivers power to its own bus 24, 26 and the BTCs 32, 34 are de-activated (opened) to keep the generator channels separated. BTCs 32, 34 may be controlled by control logic in a bus power control unit (BPCU) hereinafter referred to as a CONTROL DEVICE 46, which senses current from current transformers (CTs) 48, 50.
FIG. 2 shows the state of the electrical power system 10 when the electrical source 12 has failed. Generator control units (GCUs) 38, 40 may be used to detect the failure of either the electrical source 12 or 14 respectively. Upon failure of electrical source 12, the associated GCU 38 will isolate the electrical source 12 by commanding a generator control breaker (GCB) 42 to open, thereby removing the power source 12 from the bus bar 24.
To ensure availability of power to the loads 16, connected to the “dead bus”, the BTCs will be activated (closed) by signals from the CONTROL DEVICE 46, or by GCUs 38 and 40, as shown in FIG. 2. In this way, the unpowered bus bar 24 will be cross-fed by the active power source 14 which may supply the total power to both power bus bars 24, 26.
Likewise, in the case of a failure of power source 14, the associated GCU 40 may sense the failure and may command GCB 44 to open and thereby removing the power source 14 from the bus bar 26. CONTROL DEVICE 46 would also close both BTCs 32, 34 so that power source 12 may supply power to both power bus bars 24, 26.
FIG. 3 shows the electrical power system 10 in the situation where there has been a subsequent power bus fault. In particular, as shown in FIG. 3, power bus bar 24 has failed short-circuited; this led to the disconnection of power source 12 from the bus by its GCU. The BTCs 32, 34 may once again be de-activated (opened) to isolate the fault. Power bus bar 24 may be de-energized. The power to all loads 16 supplied by power bus bar 24 will be lost.
Some present aerospace applications have the control logic of the BTCs 32, 34 implemented in the GCUs 38, 40, while most applications have the logic implemented in the CONTROL DEVICE 46.
In applications where the electrical power system 10 has its electrical power generators operating at Constant Frequency (CF) such as that found on the Boeing 747, the system may operate with BTCs 32, 34 closed. Thus power sources 12, 14 share the burden of supplying power to the downstream loads 16, 18. Hence in a CF power system, it may not be necessary to isolate the channels in normal operation as shown in FIG. 1. In the CF power system, power source faults and power bus faults may be handled in a manner similar to the VF power system shown in FIGS. 1 and 3.
There are a number of disadvantages with the BTC control of electrical power system 10 shown in FIGS. 1-3. The control of the BTCs 32, 34 is relatively complex to insure safe power handling and transfer.
In more detail, there are two different cases which require these control algorithms.
Case 1: each power source feeds its own bus, where three control algorithms are needed as follows:
                a) control algorithm for the detection of the transfer condition/request;        b) analysis algorithm for the isolation of the cause of failure; if the generator disconnect was due to an over current fault, the closure of the BTC needs to be inhibited since this points to a bus failure that could propagate to generator 2; and        c) protection algorithm (differential fault protection—DP) to inhibit the closure of the BTC in the case a fault to ground is detected on the cable connecting between bus bar 1 and bus bar 2.Case 2: One generator feeds both busses, where two control algorithms are needed as follows:        a) control algorithm to isolate an over current fault to the specific bus; this algorithm usually involves the opening of the BTC, monitoring the over current by the GCU; with the assumption that generator 2 feeds both bus bars, if the over current disappears after the opening of the BTC, it means that the fault is on bus bar 1, therefore the BTC connection must be disabled, if the over current persists, generator 2 must be disconnected from the bus; and        b) protection algorithm (differential fault protection—DP) to open the BTC in the case of a fault to ground is detected on the cable connecting between bus bar 1 and bus bar 2.        
The implementation of the above algorithms requires use of current measurement devices, i.e. current transformers (CT), optimization for the allocation and coordination of control between GCU and CONTROL DEVICE.
The electric power system 10 shown in FIGS. 1-3 is a relatively simple example since it addresses a system including only two generating source. In practice, the electrical power system may be more complex, including multiple generators and external power sources. The principle of control remains the same; however, the control algorithms become even more complex.
As can be seen, there is a need for a simple and efficient way to handle the failure of a power source in electric power systems having multiple power sources. There is also a need for a simple and efficient way to control bus tie contactors during various failure conditions in electrical power systems.